Prediction of properties of free burning arcs including effects of ambipolar diffusion
A method is described to predict the two-dimensional distributions of temperature, velocity and potential of free burning arcs and their electrodes for cathodes of tungsten and thoriated tungsten. The effects of non-equilibrium due to the ambipolar diffusion of charged particles are included for the calculation of the plasma electrical conductivity. The electron diffusion current is explicitly included in the solution of the current continuity equation. The plasma for the arc and the electrode sheath regions is treated as a continuum, so that the thickness of the non-equilibrium regions near the electrodes is determined within the model, depending upon the arc current and the arc and electrode configuration. This new treatment allows the calculation of the negative anode fall that may occur across the anode sheath when the electron diffusion current near the anode surface becomes larger than the total arc current. For a thoriated tungsten cathode we take the work function for cooling by electron emission to be that of tungsten, as, for small percentages of thoria in tungsten, cooling effects from electrons passing through the interfaces for tungsten-thoria and then thoria-plasma will add up to be that of a tungsten-plasma interface. Calculations have been made for arcs in argon at currents between 2.5 A and 200 A. For currents above 120 A, we calculate the anode fall voltage to be negative, being -2 V at 200 A. For currents less than 50 A, non-equilibrium effects in the plasma extend across the whole arc and electron number densities can be several orders of magnitude below the values for local thermodynamic equilibrium. Calculated arc voltages, arc temperatures and electrode temperatures are in agreement with experimental measurements to within 20%.